5.3.2 Ligature definitions

The ligature information for a character in an encoding file is
optional. More than one ligature specification may be given. Each
specification looks like:

lig second-char =: lig-char

This means that a ligature character lig-char should be present in
the font for the current character (the one being defined on this line
of the encoding file) followed by second-char. You give
second-char and lig-char as character codes
(see section 3.3.3 Specifying character codes). For example, in most text
encodings (which involve Latin characters), some variation on the
following line will be present:

f lig f =: 013 lig i =: 014 lig l =: 015

This will produce a ligature in the font such that when a typesetting
program sees the two character sequence `ff' in the input, it
replaces those two characters in the output with the single character at
position octal 13 (presumably the `fi' ligature) of the font; when it
sees `fi', the character at position octal 14 is output; when
it sees `fl', the character at position octal 15 is output.

Metafont version 2 allows a more general ligature scheme; if there is a
demand for it, it wouldn't be hard to add.